fix(exam): gate Take Exam on confirmed attendance

The registrations table mirrored the server's old attendance deny-list —
ABSENT/WITHDRAWN/DISQUALIFIED — so the default REGISTERED fell through and
"Take exam" appeared before any invigilator had confirmed the candidate was
there. It is an allow-list now, PRESENT and LATE only, matching
ExamAttemptService.MAY_SIT.

A blocked candidate gets a reason rather than an empty cell: "Awaiting
attendance" with a tooltip while nobody has called the register, "Not sitting"
once attendance was taken and they are not. useExamAttempt does the same for
the direct-link path, and translates attendance_not_confirmed /
candidate_not_present instead of showing a raw error key on a Start button
that cannot work. The server refuses either way — this only makes the refusal
legible.

ScheduleExamModal no longer promises an admission number: scheduling makes a
sitting available, and the candidate registers for it themselves.

apps/backoffice/src/app/features/license-review/components/ScheduleExamModal.tsx

@@ -22,6 +22,9 @@ interface Props {
  * of a per-candidate appointment. Scoped to sittings whose certification
  * matches this application's rank, so a Chief Mate candidate cannot be seated
  * into an OOW Deck sitting by accident.
+ *
+ * Scheduling makes the sitting available; it does not register the candidate.
+ * That is their own act, from the portal's Register button.
  */
 export function ScheduleExamModal({
   opened,
@@ -61,7 +64,7 @@ export function ScheduleExamModal({
         <Text size="sm" c="dimmed">
           {t('review.scheduleExam.intro', {
             defaultValue:
-              'Assign {{applicant}} to a scheduled sitting. They will be notified with the date and their admission number.',
+              'Make a sitting available to {{applicant}}. They register for it themselves from the portal.',
             applicant: applicantName,
           })}
         </Text>
@@ -89,7 +92,7 @@ export function ScheduleExamModal({
         <Text size="xs" c="dimmed">
           {t(
             'review.scheduleExam.admissionHint',
-            'An admission number is issued automatically when the candidate is seated.',
+            'Scheduling does not seat the candidate. They must register for the sitting from the portal, and the admission number is issued then.',
           )}
         </Text>